Method of forming parts from sheet steel

ABSTRACT

A method is provided of forming a part from sheet steel. The method comprises the steps of (a) heating the sheet to a temperature at which austenitisation occurs; and (b) forming the sheet between dies into the part, father cooling the formed sheet. There is an additional step between (a) and (b) of applying cooling means to the sheet.

FIELD

The present invention relates to the forming of parts from metal. In embodiments, it relates to the forming of parts from metal sheet, such as steel and steel alloys.

BACKGROUND

Processes using “hot stamping” are emerging as preferred solutions for forming high-strength parts from steel sheet for applications in, for example, automotive “body in white” (BiW), and chassis and suspension (C&S) parts. The development of Boron steel makes such process feasible for the production of automotive safety critical panel parts, such as A-pillars, B-pillars, bumpers, roof rails, rocker rails and floor tunnels for Body-in-White and tubular parts and twist beams for C&S. The global demand for such ultra-high-strength steel parts has been growing sharply in recent years.

A typical Boron steel hot stamping process is shown in FIG. 1. Essentially it comprises the steps of:

(1) Heating the steel blank to above its austenitisation temperature, say 925° C., and soaking at that temperature to enable all the metal to be transformed into austenite. In this state the metal is soft and has high ductility (easy to form);

(2) Quickly transferring the austenitised material blank to the press;

(3) Forming the blank into the shape of the component using a cold die set, which is normally water cooled;

(4) Holding the formed part within the cold die set for a certain period (e.g. 6-10 seconds depending on geometry, sheet thickness, pressure, etc.) for quenching, enabling the hard phase of the material, e.g. martensite, (for a high strength component) to be formed; and

(5) Releasing the die when the part temperature has dropped to a sufficiently low level, say 250° C., and taking the component out.

Such a process is sometimes referred to as a “hot stamping, cold die forming and quenching” process.

Most of the heat in the work-piece goes to the die in the hot stamping process. The cooling rate is largely related to the tool surface temperature. Even if the die set is water cooled, under mass production conditions, it is difficult to keep the tool surface temperature sufficiently low. A high tool surface temperature causes the following problems:

In this conventional hot stamping process for forming complex parts from sheet steel, a sheet work-piece is transferred, as quickly as possible, from a furnace to tools at room temperature in which it is deformed and quenched simultaneously. The quench rate is sufficiently rapid to produce a martensitic microstructure in the steel, which form the basis for high strength products.

(i) The cooling rate in die quenching might become too low, which would cause undesirable soft phases to be formed in the case of steel (a low strength part produced); in the case of a light alloy, e.g. aluminium, a die quenching rate that is too low could cause undesirable grain boundary precipitation which can lead to stress corrosion cracking and a low strength part;

(ii) The cold die holding period required may be too long (because the heat transfer from the sheet is slower as a result of a warmer die, hence a greater time is required to achieve the final temperature), which reduces the productivity (increased forming cycle time);

(iii) The requirement for adequate die cooling is important, but providing it artificially (by ad hoc methods, i.e. cooling ducts with forced cooling fluid, etc.) increases tooling costs making an efficient method difficult to design and install, and can raise the tooling and maintenance costs significantly.

(iv) Tool wear and or die surface distortion are accelerated when the tool surface temperature is high, reducing tool life, the costs of which are exacerbated by ad hoc cooling systems described in (iii).

Thus, in summary, when parts are produced using this process in rapid succession, the continual contact of work-pieces from the furnace causes the temperature of the tools to increase. As a result, the quenching rate reduces, which can lead to finished products with a sub-standard microstructure. To avoid this, tool temperature can be kept low either by reducing production rate, or by using cooling systems, such as internal coolant-carrying conduits or sprays of coolant onto the tools. Often, a combination of these two methods is used to achieve a desired microstructure at the highest production rate possible for the given cooling strategy. A drawback is that all of these measures increase cost.

SUMMARY

In general terms, a two-stage cooling method is proposed to improve the productivity of high-strength sheet parts. In the proposed two-stage cooling method, the heated sheet is rapidly cooled between heating and forming. It is envisaged that this rapid cooling is by some artificial means, rather than just by ambient, still, air. For example, a high heat conductivity transfer device, an air jet or air/liquid mist spray may be used. In this way, the temperature of the blank can be reduced by the time it starts to be formed in the die. Therefore, in the forming process (in which further quenching ensues) less heat is absorbed by the tools and the rise in their temperature is reduced. Thus, maintaining a low base-line temperature is made easier, costs are reduced and productivity is increased. Other beneficial effects result from optional features.

According to a first aspect of this invention, there is provided a method of forming a part from sheet steel, the method comprising the steps of:

(a) heating the sheet to a temperature at which austenitisation occurs; and

(b) forming the sheet between dies into the part;

wherein there is an additional step between (a) and (b) of applying cooling means to the sheet to extract heat therefrom.

The additional step may include applying the cooling means to rapidly cool the sheet.

By rapidly cooling the heated sheet before forming the sheet between the dies, the sheet can be formed in the cold dies at a lower starting temperature than is conventional. This has the following effects: the sheet can cool sufficiently quickly in the dies that the hardest phase, martensite, is formed; the sheet can reach the temperature at which it is suitable for release from the dies more quickly than in the conventional process, speeding up production; the damage to tools from elevated surface temperature is reduced, increasing tool life; and reducing the need for tool cooling structures such as cooling ducts and thereby reducing the cost of the dies.

The additional step may comprise extracting heat using cooling means such as high conductivity transfer devices or by impinging cooling means such as cooling medium on the heated sheet

The cooling medium may be a fluid. It may be a gas, for example air. The cooling fluid may be a liquid, for example water. The cooling fluid of may comprise gas and liquid, for example air and water. The cooling fluid may be directed as a pressurised flow of the fluid. The cooling fluid may be directed as a jet. The cooling fluid may be directed as a mist spray. The cooling fluid may be used to cool the dies. It may be used to clean the dies. It may be used to both cool and clean the dies. The cooling fluid may be directed at the dies. It may be directed at the dies subsequently to being directed at the heated sheet and/or it may be directed simultaneously at the dies and at the heated sheet.

The cooling means may be a high heat conductivity solid, such as a copper transfer grip or plate.

The cooling means may be applied when the blank is between the dies.

The cooling between (a) and (b) may also be achieved by increasing the transfer time between the two steps, for example from the furnace to the dies.

The additional step may comprise directing the cooling fluid at the heated sheet such that the sheet is cooled sufficiently rapidly to avoid the steel entering the bainite phase. The additional step may comprise directing the cooling fluid at the heated sheet such that the sheet is cooled at more than 25° C./second on average. The additional step may comprise directing the cooling fluid at the heated sheet. The cooling fluid may be directed with duration, temperature and/or mass flow such that the sheet is cooled sufficiently rapidly to avoid the steel entering the bainite phase. The cooling fluid may be directed with duration, temperature and/or mass flow such that the sheet is cooled at more than 25° C./second on average.

The additional step may comprise directing the cooling fluid at the heated sheet such that the temperature of the sheet remains above the austenitisation temperature for the steel while being cooled in this way. The additional step may comprise directing the cooling fluid at the heated sheet such that the sheet is cooled to between 500° C. and 600° C. The cooling fluid may be directed with duration, temperature and/or mass flow such that temperature of the sheet maintains the austenitisation state for the steel while being cooled in this way. The cooling fluid may be directed with duration, temperature and/or mass flow such that that the sheet is cooled to between 500° C. and 600° C. Surprisingly, this has the effect of increasing the formability of the alloy since the strain hardening of the steel increases while the ductility remains substantially the same. The method may comprise commencing step (b) while the sheet is at a temperature at which it is in the austenite phase. The method may also comprise carrying out step (b) until the temperature of the sheet is such that it is in the martensite phase.

Step (a) may contain some or all of the features of that step of the conventional process described herein.

The method may be a method of forming parts for automotive applications. The method may be a method of forming panel parts for automotive applications. The method may be a method of forming load-bearing parts and parts adapted to bearing load in automotive applications; for example, the method may be a method of forming one or more of: pillars including A-pillars and B-pillars, bumpers, door beams, roof rails, rocker rails and floor tunnels. The method may be a method of forming Chassis and Suspension parts; for example tubular parts and twist beams.

The sheet steel may be of an alloy that contains boron.

In another aspect of the invention, a method of forming a part is provided in which the part is formed from a material other than steel. For example, the material may be an aluminium alloy. It may be in sheet form. It is therefore envisaged that the method of the first aspect may be used with aluminium alloys, for example those in sheet form. In the method of this other aspect, step (a) may comprise heating the sheet to a temperature at which a change in crystal structure substantially equivalent to austenitisation occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in schematic form an existing hot-stamping process;

FIG. 2 shows a CCT diagram for a typical Boron steel;

FIG. 3 shows a temperature profile in cold die quenching; and

FIG. 4 shows the stress-strain relationships for a Boron steel tested at temperatures of 500, 600, 700 and 800° C. at a strain rate of 1.0 s⁻¹.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

As described above, an existing method is shown in FIG. 1. A very important aspect in this existing method is that, as the hot stamped part is held in cold dies, the cooling rate should be sufficiently high, e.g. more than 25° C./second on average, as shown in FIG. 2, to enable the hardest phase of the material, martensite, to be formed. In this way, high strength components can be made. The cooling rate is not constant during the cold die quenching, as shown in FIG. 3. Initially, the temperature difference between the work-piece and the die is high and a high cooling rate can be achieved. As the work-piece temperature drops close to the tool surface temperature (which increases due to heat transfer), the cooling rate reduces significantly. In a continuous hot stamping process, the tool surface temperature can be as high as 150° C. This results in a low work-piece cooling rate in the temperature range 500° C. to 250° C. This is the sensitive range for metallurgical transformation and a low quenching rate could result in the formation of lower strength bainite instead of martensite (see FIG. 2). Thus, a low strength part would be formed.

The present embodiment provides a method in which the amount of heat transferred from the workpiece to the cold die is reduced when compared with such an existing method, thereby reducing the tool temperature in comparison with the existing method and addressing the problems of the existing method described above. This embodiment reduces the amount of heat absorbed by the die while maintaining the necessary rate of quenching, and of production.

In overview, in the present embodiment, the sheet of boron steel is rapidly cooled as it is transported from furnace to die by a solid medium of high heat conductivity, or by a fluid such as an air jet or air/liquid mist spray, and thus its temperature is reduced by the time it is placed on the die. Therefore, in the forming process (in which further quenching ensues) less heat is absorbed by the tools and the rise in their temperature is reduced. Thus, maintaining a low base-line temperature of the tools is made easier, costs are reduced and productivity is increased.

The new method involves the following steps.

First, a sheet metal blank of boron steel is heated in a furnace to above its austenitisation temperature. In the present embodiment, the blank is heated to 925° C. The blank is then soaked at this temperature to ensure the material is transformed entirely into the austenite phase. In this state the metal is soft and has high ductility (easy to form), as in the conventional process.

The next step is to transfer the austenitised material blank to the press in which it is to be formed into the shape of the part. During the transfer or, in other embodiments, after the transfer to the die but before the hot metal blank touches the die, the blank is cooled quickly by contacting it with a substance with high heat conductivity. This substance, that is this cooling means, may take the form of one, more or all of: copper grips, blowing air, directing an air-water mist or other fluid/liquid cooling medium at the blank. In the present embodiment, an air-water mist is applied to the blank. This is done by directing a fine spray of pressurized water at the blank through a plurality of nozzles. In this way, the blank is cooled to a temperature of about 600° C. The cooling rate is adjusted to be sufficiently rapid to maintain an austenite structure per the CCT diagram in FIG. 2. During this stage of work-piece cooling, it is envisaged that the same cooling medium is also used to cool and clean the tools.

The remainder of the method is the same as in the conventional method described herein. Thus, the method may be illustrated as the conventional process shown in FIG. 1, but with additional cooling during the transfer between the furnace and the die.

From the typical-stress strain curves for Boron steel shown in FIG. 4, it can be observed that when temperature decreases from 800° C. to about 600° C., the ductility of the alloy does not change very much. However, the strain hardening of the alloy leads to a near doubling of the strength. This strain hardening feature increases the formability of the alloy significantly, by causing the deformation to be more uniform (i.e. an area deformed more becomes stronger, causing deformation to occur in other areas, which then become stronger, etc.), thereby mitigating the tendency for localised necking. This is particularly important in hot stamping, since friction is normally high and the strain hardening feature could reduce the friction effects. Thus, if, as is the case in the present embodiment, a part can be formed at a temperature starting at about 600° C. in the dies, rather than 800° C. as is done conventionally; more complex-shaped components can be formed. It should be emphasized that this effect cannot be achieved by simply heating the sheet to a lower initial temperature, as it must first be fully austenitised.

The CCT diagram for Boron steel in FIG. 2 shows that the alloy is still in the austenite state if it is cooled quickly to about 500-600° C. If the cooling is too slow, the lower-strength bainite phase begins to form; the present method, however, avoids this. In the present method, as the blank is transferred to cold dies while in this temperature range and maintained at a temperature between 450-500° C. during the entire forming process, all phase transformation takes place during the cold die holding period and the austenite is entirely converted to martensite to produce a high strength part.

In existing methods, the formed part is released from the die as soon as the part temperature drops to about 250° C. At this temperature, phase transformation has been completed and no obvious thermal distortion is observed by further cooling in the air without the tool constraint. The cold die quenching period (i.e. the time for which the part is held in the die) required to cool a part from about 800° C. to about 250° C. (550° C. difference), is about 5 to 15 seconds in these existing methods, depending on the thickness and shape of the work-piece and part shape. Thus, a significant amount of heat has to be absorbed by the die directly, which makes cooling the die difficult.

In the present embodiment, the part is formed at about 600-500° C. Thus, in the cold die quenching period, the only need is to bring the part temperature down from, at the lower end of this range, 500° C. to about 250° C. (250° C. difference). Only about half the amount of heat therefore needs to be extracted from the die, and so the cooling requirement for the tool is much lower. The tool design can therefore be simpler and the tool can be cheaper. The lower temperature of the tool surface reduces the cold die holding period significantly, and also increases the cooling rate significantly during the temperature range of 500° C. to 250° C. The holding time can be reduced to about 2 to 8 seconds. Thus, productivity can be increased significantly. This is vital for, for example, a competitive automotive company. In addition, the lower tool surface temperature reduces tool wear, thus increasing tool life significantly, which is an additional benefit for reducing production costs. 

1. A method of forming a part from sheet steel, the method comprising the steps of: (a) heating the sheet to a temperature at which austenitisation occurs; and (b) forming the sheet between dies into the part, further cooling the formed sheet; wherein there is an additional step between (a) and (b) of applying cooling to the sheet.
 2. A method according to claim 1, wherein the additional step takes place before the heated sheet is placed between the dies.
 3. A method according to claim 1, wherein the additional step takes place while the heated sheet is placed between the dies.
 4. A method according to claim 1, wherein the cooling comprises a cooling fluid such as a gas, for example air; and/or wherein the cooling fluid comprises a liquid, for example water, the method comprising directing the cooling fluid at the heated sheet.
 5. A method according to claim 1, wherein the cooling fluid is directed as a pressurised flow of the fluid.
 6. A method according to claim 1, wherein the cooling fluid is directed as a jet and/or is directed as a mist spray.
 7. A method according to claim 1, wherein the cooling fluid is directed by controlling the duration, temperature and/or mass flow of the cooling fluid.
 8. A method according to claim 1, wherein the cooling comprises cooling plates, for example cool copper plates.
 9. A method according to claim 1, wherein the cooling can be achieved by increasing the transfer time with natural air cooling.
 10. A method according to claim 1, wherein the additional step comprises directing the cooling fluid at the heated sheet such that the sheet is cooled sufficiently rapidly to avoid the steel entering the bainite phase.
 11. A method according to claim 1, wherein the additional step comprises directing the cooling fluid at the heated sheet such that the sheet is cooled at more than 25° C./second on average.
 12. A method according to claim 1, wherein the additional step comprises directing the cooling fluid at the heated sheet such that the temperature of the sheet remains above the austenitisation temperature for the steel.
 13. A method according to claim 1, wherein the additional step comprises directing the cooling fluid at the heated sheet such that the sheet is cooled to between 500° C. and 600° C.
 14. A method according to claim 1 and comprising the further step of directing cooling fluid to cool the dies.
 15. A method according to claim 1 and comprising the further step of directing cooling fluid to clean the dies.
 16. A method according to claim 1, comprising the further step of directing cooling fluid to cool and clean the dies in the same step. 